<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    html {
        --c: blue;
    }

    body[data-theme='test'] {
        --c: red;
    }

    div {
        color: var(--c);
    }
</style>

<body data-theme="test">
    <div>阿萨德了发束带结发</div>
</body>

</html>